Abbreviations: 5-HTP, 5-hydroxytryptophan; AD, Alzheimer’s disease; APOE, apolipoprotein E; GABA, gamma-aminobutyric acid; l-DOPA, l-3,4-dihydroxy-phenylalanine; LC-MS/MS, liquid chromatography tandem mass spectrometry; MCI, mild cognitive impairment; MPTP, methyl-4-phenyl-1,2,3,6- tetrahydropyridine; NMDA, N-methyl-D-aspartate; SadA, staphylococcal aromatic amino acid decarboxylase; SCFA, short chain fatty acid; TPH1, tryptophan hydroxylase 1. Here, TPH1 is linked to Alzheimer disease.